Currently, sense of confidentiality on techniques and information has gained more attention than ever before. Common ways to maintain secrecy include, for example, encrypting files or a display device for operating the files.
However, with the development of computer skills, encrypted files or encrypted devices can often be cracked. Moreover, secret-stealers are likely to photograph a display screen of an operating device (e.g., a display device such as a computer or the like) to obtain contents of the encrypted files while an operator is operating encrypted files.
As a result, how to prevent leakage when operating confidential files has become an urgent technical problem to be solved in the field.